DLP pioneer tells how TI did it with mirrors

Solid-state physicist and Texas Instruments Inc. fellow Larry Hornbeck won an Emmy for his invention of the digital micromirror device (DMD), the microelectromechanical system at the heart of TI's digital light processing (DLP) technology for projection displays. In addition to advancing the state of the art for digital cinema, front projectors and HDTV, Hornbeck's MEMS micromirror is enabling 3-D metrology systems that measure with finer detail, confocal microscopes that eliminate the out-of-focus "haze" normally seen around fluorescent samples, and holographic storage systems that write data in three dimensions instead of just two. Hornbeck told EE Times' R. Colin Johnson that TI has still more applications up its sleeve for digital micromirrors.

EE Times: Texas Instruments has perhaps the industry's longest history in MEMS development.Larry Hornbeck: TI's history with MEMS goes back to 1977.

EE Times: Back then, you couldn't have been aiming at digital light processing. Were you even focusing on an application?Hornbeck: Even back then, we were interested in using MEMS to modulate light. I had only been at TI for a few years after receiving my PhD from Case Western University as a solid-state physicist. We got started in MEMS back then because of a Defense Department contract to make a spatial light modulator using deformable mirrors. This was an analog technology that the DOD wanted to use for optical computing.

EE Times: I guess you had to invent the fabrication techniques you needed to even get started with MEMS.Hornbeck: In 1981, MEMS meant bulk micromachining of single-crystal silicon, which made the devices expensive to manufacture. But the universities were already experimenting with surface micromachining of polysilicon, which was much more economical and has become the traditional way of doing MEMS today.

EE Times: As your micromirrors became more successful, did you dedicate a fab to MEMS, as Analog Devices did in Cambridge [Mass.]?Hornbeck: We've never needed a special fab but have always made our MEMS fabrication compatible with our conventional CMOS manufacturing areas. We finish out all the transistors and metallization layers for interconnecting the transistors, and then we use a low-temperature process to put MEMS on top of the completed CMOS chip.

EE Times: I see; you finish the whole chip but leave an area open to add the MEMS last.Hornbeck: That's a radical departure from the way others do MEMSI believe we are still the only company that does it this way. The way I implemented the process was by choosing aluminum alloys for the mechanical elements and conventional photoresist to act as a sacrificial spacer. All of this is done at temperatures under 200°C so that metallization is not affected, the transistors are not affected, none of the finished CMOS circuitry is affected when we add MEMS to a chip. To this day, this forms our standard basis of manufacturing MEMS micromirrors, but it was a radical departure at the time.

EE Times: Only Bosch Sensortec and its spin-off SiTime seem to put their MEMS structures down firstusing high-temperature processing, which they swear by. But most of the other MEMS startups, such as Discera and Silicon Clocks, have gone to putting the MEMS down lastusing either polysilicon or, in Silicon Clocks' case, silicon germanium. And whether they favor MEMS-first or MEMS-last, all the MEMS vendors seem to agree that the holy grail is seamless integration of MEMS structures onto the same CMOS chips as the circuitry to which they interface. Akustica claims to be there already with its MEMS microphone, though they do add a step at the end to remove sacrificial oxide layers. All told, I think you were way ahead by integrating MEMS with CMOS from the beginning.Hornbeck: We think so. This is one of the pillars we are resting on as a basis for our success in DLP.